Steel for production of high-strength components with excellent low-temperature toughness and uses of a steel of this type

ABSTRACT

The invention provides a high-strength steel that has excellent ductile fracture values J integral even at low temperatures, so the risk of the component, which is produced in each case from the steel, breaking is reduced to a minimum even under unfavourable, hard operating conditions. This is achieved in that it contains (in % by weight) 0.08 to 0.25% C, 0.10 to 0.30% Si, 0.80 to 1.60% Mn,=0.020% P,=0.015% S, the sum of the P and S content being=0.030%, 0.40 to 0.80% Cr, 0.30 to 0.50% Mo, 0.70 to 1.20% Ni, 0.020 to 0.060% Al, 0.007 to 0.018% N,=0.15% V,=0.07% Nb, the sum of the V and Nb content being=0.020% and the remainder being iron and inevitable impurities. The steel according to the invention is particularly suitable for the manufacture of high-strength chains.

The invention relates to a steel for the production of high-strengthcomponents with excellent low-temperature toughness. Steels of this typeare used, for example, for the manufacture of stop or clamping means,such as are required for fastening and securing loads. These steels are,in particular, processed to form hot-rolled bar steel, rolled wire orbright steel, from which welded round steel chains are then made.

The demands placed on steels of the aforementioned type are set out inDIN 17 115. In addition to good deformability and equally goodsuitability for welding, the steels have to have excellent strength andtoughness properties in order to meet the requirements that are set as aresult of the stresses that occur in practice.

The high-grade steels 23 MnNiCrMo 5 3 and 23 MnNiCrMo 54, which areknown for this purpose and are specified in DIN 17 115, respectivelycomprise (in % by weight) 0.20 to 0.26% C, =0.25% Si, 1.10 to 1.40% Mn,respectively 0.020% P and S, the sum of the contents of P and S notexceeding 0.035%, if necessary 0.020 to 0.050% Al, up to 0.014% N and0.40 to 0.60% Cr. 0.20 to 0.30% Mo and 0.70 to 0.90% Ni are also addedto the steel 23 MnNiCrMo 5 2, whereas the steel 23 MnNiCrMo 5 4additionally contains 0.50 to 0.60% Mo and 0.90 to 1.10% Ni.

Another steel for the manufacture of chains intended for securing ormooring ships or drilling platforms is known from Chinese patentpublication CN-1281906. The Abstract of this publication, which isavailable in the WPINDEX database, discloses that the known steelcontains (in % by weight) 0.25 to 0.35% C, 0.15 to 0.30% Si, 1.45 to1.75% Mn, 0.90 to 1.40% Cr, 1.00 to 1.20% Ni, 0.45 to 0.65% Mo, 0.02 to0.06% Nb, 0.020 to 0.05% Al, up to 0.020% P, up to 0.15% S, up to 0.20%Cu, up to 0.03% Sn, up to 0.01% Sb, up to 0.04% As, up to 0.005% B, upto 0.009% N, up to 0.0020% 0, up to 0.0002% H, the remainder being Feand inevitable impurities, wherein a carbon equivalent also has to begreater than 1.4.

Practical experience has shown that although the known steels meet therequirements set with respect to strength and toughness at ambienttemperature, problems arise, especially with regard to toughness, atlower temperatures.

The object of the invention was therefore to provide a high-strengthsteel that has excellent toughness even at low temperatures, so the riskof the component, which is produced in each case from the steel,breaking is reduced to a minimum even under unfavourable, hard operatingconditions. Advantageous uses of this steel are also to be specified.

With regard to the steel, this object is achieved, according to theinvention, in that steel according to the invention for the productionof high-strength components with excellent low-temperature toughness hasthe following composition (in % by weight):

-   C: 0.08 to 0.25%,-   Si: 0.10 to 0.30%,-   Mn: 0.80 to 1.60%,-   P:=0.020%,-   S:=0.015%,    the sum of the P and S content being=0.030%,-   Cr: 0.40 to 0.80%,-   Mo: 0.30 to 0.50%,-   Ni: 0.70 to 1.20%,-   Al: 0.020 to 0.060%,-   N: 0.007 to 0.018%,-   V:=0.15%,-   Nb:=0.07%,    the sum of the V and Nb content being=0.020%, the remainder being    iron and inevitable impurities.

In the case of steel according to the invention, the individual alloycomponents are selected such that a property profile that optimallysatisfies the requirements set is achieved. This is achieved by thecontents of Cr, Ni and N that are set according to the invention and theminimum sum of the contents of Nb and V. If the content ranges of thesealloy elements that are set according to the invention are adhered to, aparticularly high toughness, good hardenability, improved retention ofhardness when tempering and a particularly fine grain structure areachieved. Steel according to the invention is also highly cold formableand has high strength in the finished processed state. It is alsodistinguished by high notch impact toughness and a low fractureappearance transition temperature such that brittle fracture occurs onlyat temperatures that are substantially lower than the brittle fracturetemperature of steels known from the prior art.

The C-contents located in the range from 0.08 to 0.25% by weight ensurethe good low-temperature resistance of steels according to theinvention. Particularly positive results are produced in this connectionif the C content is from 0.16 to 0.23% by weight.

The good hardenability and retention of hardness when tempering of thesteel according to the invention are achieved by means of the limitationof the Cr contents to 0.40 to 0.80% by weight in combination with Mocontents from 0.30 to 0.50% by weight. The degree of certainty withwhich this combined effect is achieved may be increased in that the Crcontents are adjusted to 0.40 to 0.65% by weight and the Mo contents to0.35 to 0.50% by weight.

Ni contents from 0.70 to 1.20% by weight, in particular 0.75 to 1.00% byweight, bring about the good low-temperature toughness that is to beemphasised, in particular, in steel according to the invention.

The contents of Al from 0.020 to 0.060% by weight, in particular 0.020to 0.045% by weight, and of N from 0.007 to 0.018% by weight, inparticular 0.007 to 0.015% by weight, lead in steels according to theinvention to a particularly fine grain structure.

Finally, the fact that steel according to the invention contains intotal at least 0.02% by weight Nb and V, while the contents of V arelimited to at most 0.15% by weight and of Nb to at most 0.07% by weight,ensures that the desired fine grain structure is still maintained evenat elevated temperatures. It has surprisingly been found in thisconnection that this effect occurs particularly reliably if the steelaccording to the invention is free of vanadium.

According to a preferred configuration, there is therefore no Vwhatsoever in steel according to the invention, or it is present only asan inevitable impurity.

The fine grain remains stable even in the course of the temper-hardeningtreatment. Finished processed steel according to the invention thereforecommonly has an austenite grain size that is finer than ASTM 10. Thefineness of the structure of steel according to the invention istherefore substantially greater than that of known steels, for which anaustenite grain size of ASTM 5 is required according to DIN 17 115.

The invention therefore provides a steel that has excellent toughnesseven at low temperatures. As a result of the favourable combination ofits properties, the risk of a component produced from steel according tothe invention breaking is reduced to a minimum even under unfavourable,hard operating conditions.

Steel according to the invention is preferably processed to rolledsteel. The aim of the processing is to preserve via each of theprocessing steps the finest possible grain structure of the steelaccording to the invention. This includes not only the process stepscarried out during the heating and rolling, but also the annealingtreatments that are carried out before and after the componentformation. According to the invention, the heating and rollingconditions are thus selected such that despite the diffusion processesthat occur during heating, high rolling temperatures may be avoided, inorder to suppress the formation of coarse grain. The temperatures duringthe further deformation are also selected by means of a controlledwithdrawal of energy during the heat deformation such that the desiredconstruction with its fine grain structure is maintained. An acceleratedwithdrawal of heat immediately after the final deformation stepprevents, in the sense of a “freezing” of the ultimately achievedstructure state, undesirable precipitation processes that wouldotherwise result in a decrease in the hardness and toughness. Instead,by a long-term heat treatment desired precipitation states of thecarbonitrides with respect to the size and distribution thereof areproduced, in order to obtain the relatively low material strengths ofthe steel in the hot-rolled state that is desired for cold forming ofthe steel to form the respective component.

As a result of its particular property spectrum, steel according to theinvention is particularly suitable for the production of high-strengthcomponents by cold forming with subsequent temper-hardening. Thesecomponents may, for example, be means for the carrying, pulling,lifting, conveying or securing of loads that are allocated to thehighest strength class. Such articles, which may be summarised under thegeneral term “stop and clamping means”, include, for example, attachmentpoints, hooks, clips, eyelets, chains, joints, catch elements, rockers,braces, spindle and ratchet clamps, attachment eyes and the like.

Means for the connection of structural elements having excellent useproperties may also be made from steel according to the invention. Thesestructural elements are, for example, bolts or other connecting or forcetransmission elements, such as screws, clamps, rods or the like.

One field of application for which steel according to the invention isparticularly suitable is the manufacture of chains. Chains made of steelof the composition according to the invention reliably withstand heavyloads even at very cold temperatures, without any risk of fracture orcomparable damage. Round steel chains, especially welded round steelchains, which are able to satisfy even the most stringent requirements,may thus be made of steel according to the invention.

The components made of steel according to the invention commonly have astrength of at least 1,200 MPa, in particular more than 1,550 MPa, 1,600MPa or 1,650 MPa. It should be emphasised in this connection that at astrength of at least 1,550 MPa, the fracture appearance transitiontemperature FATT of the components made of steel according to theinvention is commonly at most −60° C. This limit temperature issignificantly lower than in known steels.

It is equally notable that the notch impact working value, in the caseof components produced from steel according to the invention, iscommonly more than 45 J and the respective component has a technicalcrack initiation toughness J_(IC) of more than 170 N/mm at −60° C., inparticular more than 185 N/mm. The crack initiation toughness J_(IC) isa value defined in the ASTM 1820 that allows evaluation of the ductilefracture tendency of a steel material.

The high degree of toughness of the steel according to the invention isalso discernible in the fact that components produced from steel of thistype commonly exhibit an elongation at break of more than 28%.

The invention will be described below in greater detail with referenceto an embodiment.

A steel comprising (in % by weight)

-   0.19% C,-   0.20% Si,-   1.31% Mn,-   0.005% P,-   0.010% S,    P content+S content=0.015%,-   0.45% Cr,-   0.37% Mo,-   0.88% Ni,-   0.400% Al,-   0.008% N,-   0.01% V,-   0.06% Nb,    (V content+Nb content=0.07%),    the remainder being Fe and inevitable impurities,    was melted and processed to form a rolled steel. In order to ensure    the finest possible grain structure of the obtained product after    the hot rolling, the rolling temperatures were kept at a low level    during the hot rolling. Moreover, cooling of the rolled product was    carried out between each rolling step, in order to dissipate heat    produced by the hot forming itself. Immediately after the hot    rolling, the obtained hot-rolled product was quenched in order to    freeze the fine grain structure of the steel that is present on    leaving the hot rolling path such that it is also reliably preserved    in the subsequent processing steps.

After the hot rolling and a long-term heat treatment, which was carriedout in order to set a beneficial strength for the subsequent coldforming, the rolled steel was shaped to form chain links, which wereclosed by welding once the chain had been assembled.

The chains produced in this manner exhibited a fine grain structure ofASTM 11, a strength of 1,270 N/mm² and a fracture appearance transitiontemperature FATT determined at this strength of −70° C. Their notchimpact working value was 557 J at −60° C. test temperature and theelongation at break was 28%.

In the accompanying diagram, the course of the ductile fracture value Jintegral for steel according to the invention is plotted over the crackexpansion REW at a temperature of −60° C. for a standardised initialcrack length of 0.4. It may be seen that there is a crack initiationtoughness J_(IC) of 185 N/mm² at the technically relevant start hestable crack expansion.

1. Steel for the production of high-strength components with excellentlow-temperature toughness, having the following composition (in % byweight): C: 0.08 to 0.25%, Si: 0.10 to 0.30%, Mn: 0.80 to 1.60%,P:=0.020%, S:=0.015%, the sum of the P and S content being=0.030%, Cr:0.40 to 0.80%, Mo: 0.30 to 0.50%, Ni: 0.70 to 1.20%, Al: 0.020 to0.060%, N: 0.007 to 0.018%, V:=0.15%, Nb:=0.07%, the sum of the V and Nbcontent being=0.020%, the remainder being iron and inevitableimpurities.
 2. Steel according to claim 1, wherein its C content is from0.16% by weight to 0.23% by weight.
 3. Steel according to claim 1,wherein its Mn content is from 1.00% by weight to 1.35% by weight. 4.Steel according to claim 1, wherein its Cr content is from 0.40% byweight to 0.65% by weight.
 5. Steel according to claim 1, wherein its Mocontent is from 0.35% by weight to 0.50% by weight.
 6. Steel accordingto claim 1, wherein its Ni content is from 0.75% by weight to 1.00% byweight.
 7. Steel according to claim 1, wherein its Al content is from0.020% by weight to 0.045% by weight.
 8. Steel according to claim 1,wherein its N content is from 0.007% by weight to 0.015% by weight. 9.Steel according to claim 1, wherein it has an austenite grain size thatis finer than ASTM
 10. 10. Use of a steel composed according to claim 1for the production of high-strength components by cold forming withsubsequent temper-hardening.
 11. Use according to claim 10, wherein thecomponents are means for the carrying, pulling, lifting, conveying orsecuring of loads.
 12. Use according to claim 10, wherein the componentsare means for the connection of structural elements.
 13. Use accordingto claim 10, wherein the components are chains.
 14. Use according toclaim 13, Wherein the chains are round steel chains.
 15. Use accordingto claim 13, wherein the chains are welded.
 16. Use according to claim10, wherein the components have a strength of at least 1,200 MPa. 17.Use according to claim 16, wherein the strength is at least 1,550 MPa.18. Use according to claim 16, wherein the strength is at least 1,600MPa, in particular at least 1,650 MPa.
 19. Use according to claim 10,wherein at a strength of at least 1,550 MPa, the fracture appearancetransition temperature FATT of the components is at most −60° C.
 20. Useaccording to claim 10, wherein the notch impact working value is morethan 45 J.
 21. Use according to claim 10, wherein the material of thecomponent has a technical crack initiation toughness J_(IC) of more than170 N/mm².
 22. Use according to claim 21, wherein the technical crackinitiation toughness J_(IC) is more than 185 N/mm².
 23. Use according toclaim 10, wherein the components exhibit an elongation at break of morethan 28%.